1. Field of the Invention
The present invention relates to a method and apparatus for recording scrambled digital data, for example television broadcasts.
2. Background Art
Transmission of encrypted data is well-known in the field of pay TV systems, where scrambled audiovisual information is broadcast typically by satellite to a number of subscribers, each subscriber possessing a decoder or integrated receiver/decoder (IRD) capable of descrambling the transmitted program for subsequent viewing.
In a typical system, scrambled digital data is transmitted together with a control word for descrambling of the digital data, the control word itself being encrypted by an exploitation key and transmitted in encrypted form. A decoder receives the scrambled digital data and encrypted control word which uses an equivalent of the exploitation key to decrypt the encrypted control word and thereafter descramble the transmitted data. A paid-up subscriber will receive periodically the exploitation key necessary to decrypt the encrypted control word so as to permit viewing of a particular program.
With the advent of digital technology, the quality of the transmitted data has increased many times over. A particular problem associated with digital quality data lies in its ease of reproduction. Where a descrambled program is passed via an analogue link (e.g. the “Peritel” link) for viewing and recording by a standard VCR the quality remains no greater than that associated with a standard analogue cassette recording. The risk that such a recording may be used as a master tape to make pirate copies is thus no greater than with a standard shop bought analogue cassette.
By way of contrast, any descrambled digital data passed by a direct digital link to one of the new generation of digital recording devices (for example, a DVHS recorder) will be of the same quality as the originally transmitted program and may thus be reproduced any number of times without any degradation of image or sound quality. There is therefore a considerable risk that the descrambled data will be used as a master recording to make pirate copies.
French Patent Application 95 03859 shows one way of overcoming this problem, by means of a system in which descrambled digital data is never allowed to be recorded on the digital recording medium. Instead, the decoder described in this application forwards the data for recording on the support medium in its scrambled form. The control word necessary to descramble the data is re-encrypted by means of another key and stored on the recording support with the scrambled data. This new key is known only to the receiver/decoder and replaces the exploitation key needed to obtain the control word for viewing of the program.
The advantage of such a system is that the data is never stored in a “clear” form and cannot be viewed without possession of the new key, stored in the decoder. The system also possesses the advantage that, since the exploitation key changes on a monthly basis, the use of a key chosen by the decoder to re-encrypt the control word registered on the digital tape means that the decoder will still be able to decrypt the control word recorded on the tape even after the end of a subscription month.
The disadvantage of the system propose in this previous patent application is that the recording can only be viewed in conjunction with that particular decoder. If that decoder breaks down, or is replace, the recording can no longer be replayed. Equally, it is not possible to play the recording directly in a digital recorder without connecting the decoder in the system.